Osteotome

ABSTRACT

A tool ( 1, 31 ) for cutting bone comprises an elongate blade ( 7,37 ) connectable to a generator of longitudinal-mode ultrasonic vibrations. The blade ( 7,37 ) has two lateral cutting edges ( 9,10 ) linked by a rounded distal tip ( 8 ). A series of triangular teeth ( 13 ) extends along each cutting edge ( 9,10 ) and the distal tip ( 8 ). The blade ( 37 ) may taper towards each cutting edge ( 9,10 ) and the distal tip ( 8 ). A variant of the tool ( 21 ) comprises an elongate part-cylindrical blade ( 27 ) connectable to a generator of torsional-mode ultrasonic vibrations. The blade ( 27 ) has a cutting edge at its distal tip ( 28 ) provided with a plurality of triangular teeth ( 23 ). All forms of the tool ( 1, 21, 31 ) are particularly suitable for cutting cancellous bone around an implant to be removed during revision of a joint arthroplasty.

The present invention relates to a tool for cutting into or throughbone, for example during orthopaedic surgery. More particularly, but notexclusively, it relates to a tool for cutting through cortical orcancellous bone, for example to separate a joint prosthesis fromsurrounding bone as part of a revision procedure.

A frequently required procedure in orthopaedic surgery is revision of ajoint arthroplasty, for example revision of a hip joint replacement,should an implanted prosthesis break or wear unacceptably over itsarticulating surface. The invention will be described in relation to itsuse in hip joint revision, but is equally applicable to other joints andthe terms “hip”, “pelvis” and “femur” may be replaced as necessary. Inmany cases, an implanted prosthesis is secured in a cavity within abone, such as a femur, using polymeric organic cement such aspolymethylmethacrylate. Tools have been devised to soften and removethis cement and to allow convenient removal of a worn or damagedprosthesis, followed by implantation of a replacement.

However, there has been a recent increase in the use of press-fitprostheses. No cement is used to hold these in place within the femur,pelvis, etc. Instead, the implanted portions of the prostheses haveporous surfaces or surfaces coated with hydroxy-apatite, which encourageingrowth of bone, leading to stable, well-anchored implants. Thisformation of cancellous bone may also occasionally occur withcement-anchored implants. While not as strong as the structural bone ofthe wall of the femur, the pelvis or other bone, cancellous bone is noteasily susceptible to cutting using the tools devised for revisingcement-anchored implants, and it has become necessary to attackcancellous bone mechanically in order to revise such implants.

Furthermore, in order to remove a prosthesis, it may be necessary toremove portions of cortical bone, which cannot be achieved without usingmechanical means.

A manual osteotome is effectively a specialised form of chisel, which isforced longitudinally through the bone between a prosthesis andsurrounding structural bone. The force required can be so great as tocompromise the directional accuracy of the technique, and may therebydamage surrounding structural bone, especially if it is weakened byosteoporosis or the like.

Another approach is to use powered burrs to drill out the bone. Thesemay also be difficult to guide accurately, and flexure in their elongaterotating drive shafts may lead to unacceptable collateral damage insurrounding structural bone. This approach also produces inconvenientlylarge quantities of bone swarf, which must be removed to allow clearvisualisation of the point at which the burr is cutting. Furthermore,high-speed burrs lead to significant localised frictional heating, whichmay also harm adjacent bone, tissue or marrow. Manual sawing throughbone is a slow, tiring process, also leading to localised heating andcopious bone swarf. In any case, conventional bone saws could not easilybe inserted or operated between a hip or other joint prosthesis shaftand an inner wall of a femur, or between a part-spherical acetabularshell and a pelvic bone, for example.

It is hence an object of the present invention to provide a tool forcutting bone, particularly bone adjacent an arthroplasty implant, thatobviates the above disadvantages and allows accurate, rapid andconvenient removal of such implants as part of a revision procedure.

According to a first aspect of the present invention, there is provideda tool for cutting bone comprising a means of generating ultrasonicvibrations, elongate blade means operatively connectable thereto andhaving at least one cutting edge provided with a plurality of serrationseach having a first cutting facet substantially transverse to thedirection of said ultrasonic vibrations.

In a first embodiment, the tool is adapted to be vibrated bylongitudinal mode ultrasonic vibrations, for example directedsubstantially parallelly to a longitudinal axis of the blade means.

Preferably, the blade means then comprises an elongate substantiallyplanar member having two substantially oppositely-facing lateral edges.

Advantageously, said lateral edges each extend substantially parallellyto the longitudinal axis of the blade means.

The elongate member may further comprise a rounded distal tip. Saiddistal tip may extend between a distal end of a first said lateral edgeand a distal end of a second said lateral edge.

Preferably, the cutting edge extends along at least part of a respectiveone of said lateral edges.

Advantageously, the cutting edge extends along at least part of eachsaid lateral edge.

The cutting edge may extend around all or part of the rounded distaltip.

A continuous cutting edge may extend around at least a distal portion ofeach lateral edge and the distal tip extending therebetween.

Preferably, at least part of the blade means has a cross-sectionalprofile tapering towards one or each lateral edge.

Advantageously, the blade means has a cross-sectional profile adjacentits distal tip tapering towards said tip.

Said tapering profile may comprise at least one angled surface locatedon each opposite face of the blade means.

A single angled surface may extend adjacent each lateral edge and thedistal tip on each said face.

Said angled surfaces may be connected at their respective outerperipheries by an edge surface extending transversely to the generalplane of the blade member.

Said edge surface may be substantially narrower than an overallthickness of the blade means.

Preferably, said tapering profile is at least coextensive with thecutting edge of the blade means.

Preferably, each serration of the or each cutting edge is generallytriangular.

Each serration may have the first, cutting facet extending generallyorthogonally to a local alignment of the cutting edge.

Each serration may then have a second, angled facet extending obliquelyto said local alignment.

Each pair of neighbouring serrations may be so relatively aligned that afirst facet of one serration of said pair is adjacent a second facet ofthe next serration of said pair.

Each first facet of a first lateral cutting edge may face towards adistal tip of the blade means and each first facet of a second oppositelateral cutting edge may then face towards a proximal root of the blademeans.

Preferably, each serration extends outwardly from the cutting edge,substantially in the plane of the elongate member.

In a second embodiment, the tool is adapted to be vibrated by torsionalmode ultrasonic vibrations.

The blade means then preferably comprises an elongate member having acurved cross-section, optionally substantially comprising an arc of acircle.

Advantageously, said cross-section is substantially constant along awhole of the elongate member.

The tool may be so adapted as to be torsionally vibratable about anlongitudinal axis extending through the centre of said circle.

Preferably, a distal tip of the elongate member comprises the cuttingedge of the tool.

The serrations of the cutting edge may be generally triangular.

The serrations may extend distally from the tip of the member.

A portion of the elongate member adjacent its tip may taperlongitudinally towards said tip.

The tapered portion may comprise an angled surface located on a concaveface of a curved elongate member.

In each embodiment, the generator means is advantageously adapted togenerate ultrasonic vibrations at a frequency within the range of twentyto seventy-five kilohertz.

According to a second aspect of the present invention, there is provideda method of cutting bony material comprising the steps of providing atool as described in the first aspect above, applying a cutting edge ofthe tool to a surface of bony material to be cut, causing the tool tovibrate at an ultrasonic frequency and drawing the cutting edge of thetool across said surface.

Preferably, the cutting edge is drawn reciprocally across said surface.

Advantageously, the bony material comprises cancellous and/or corticalbone holding an orthopaedic implant to a bone of a living body, and themethod comprises the step of cutting the bone as described above untilthe implant is separable therefrom.

Embodiments of the present invention will now be more particularlydescribed by way of example and with reference to the accompanyingdrawings in which:

FIG. 1 is a perspective view of a first tool embodying the presentinvention;

FIG. 2A is a plan view of a distal portion of the tool shown in FIG. 1;

FIG. 2B is a schematic plan view of an intermediate part of the distalportion shown in FIG. 2A;

FIG. 3A is a partial perspective view of a second tool embodying thepresent invention;

FIG. 3B is an elevation of a distal end of the tool shown in FIG. 3A;

FIG. 4 is a perspective view of a third tool embodying the presentinvention;

FIG. 5 is a cross-sectional elevation of a blade of the tool shown inFIG. 4, taken along the line V-V; and

FIG. 6 is a cross-sectional elevation of a blade of the tool shown inFIG. 4, taken along the line VI-VI.

Referring now to the Figures and to FIG. 1 in particular, a firstosteotomy tool 1 comprises a cylindrical connecting body 2 provided at aproximal end with a threaded spigot 3, by which the tool 1 maydetachably be connected to a generator of ultrasonic vibrations (notshown). An elongate blade portion 4 of the tool 1 extends from a distalend of the connecting body 2, and is aligned generally coaxiallytherewith.

The blade portion 4 comprises a proximal blade root 5 having asubstantially rectangular cross-section and linked by a tapered portion6 to a thin, flat elongate blade 7 with a generally rounded distal tip8. A distal portion of the blade 7 has two oppositely-facing lateralcutting edges 9, 10. Each of the lateral cutting edges 9, 10 and the tip8 is provided with a plurality of teeth 13, as shown in more detail inFIGS. 2A and 2B. A proximal portion of the blade 7 is toothless,although the relative lengths of the toothed and toothless portions mayvary from that shown.

The cylindrical connecting body 2 is provided with spanner flats 11 toallow application of tightening torque sufficient to bring the tool 1into secure contact with the ultrasound generator, allowing effectivevibrational coupling through a contact surface 12 of the body 2. Thetool 1 is preferably made of titanium or stainless steel.

As FIGS. 2A and 2B illustrate, the teeth 13 of the blade 7 arepreferably shaped generally as conventional saw teeth, having a firstedge 14 substantially orthogonal to a longitudinal axis of the blade 7and a second edge 15 at a relatively shallow angle thereto. In aconventional saw, the first edge 14 would be sharpened, and the sawwould cut when pulled (or sometimes pushed) in a longitudinal directionin which the first edge 14 is a leading edge of the tooth 13. In thepresent invention, it is believed to be unnecessary to sharpen the teeth13.

In the tool 1 shown, the teeth 13 extend in a continuous array along afirst cutting edge 9, around the tip 8 and along a second cutting edge10, without the relative dispositions of the first and second edges 14,15 of the teeth 13 changing. Thus, the first cutting edge 9 is adaptedto cut on a longitudinal pull stroke as indicated by arrow 16 and thesecond cutting edge 10 is adapted to cut on a longitudinal push strokeas indicated by arrow 17.

Were the tool 1 a conventional mechanical saw, this arrangement wouldnot be particularly effective, a push cut being particularly difficultto control in direction or force. Manual sawing at bone, even cancellousbone, produces significant frictional heating and requires considerableeffort on the part of the user.

However, when the blade 7 is subjected to longitudinal mode ultrasonicvibrations, directed parallelly to the longitudinal axis 18 of the tool1, the effectiveness of both the pull stroke 16 and the push stroke 17is greatly improved. The velocity amplitude of the first edge 14 of eachtooth 13 as it contacts the bone is much greater than the speed of thestroke 16, 17 alone. This leads to much more rapid cutting through thebone, with much less friction, and hence much less heating. The userdoes not need to force the tool 1 through the bone, allowing muchgreater accuracy and control in the cut, for both the push and pullstrokes 16, 17. The tip 8 may be sunk longitudinally into the bone withonly small lateral movements of the tool 1.

The tool 1 is connected to an ultrasound generator operating in thefrequency range 20-75 kHz.

Thus, for a replacement hip joint prosthesis held in a cavity within afemur by friction or by interaction with cancellous bone, and requiringrevision, it is relatively straightforward to sink the tool 1 betweenthe stem of the prosthesis and the femur itself, tip first and extendinggenerally parallelly to the stem. The tool 1 can then be moved laterallyaround the stem, with a gentle sawing motion, cutting through the boneand freeing the prosthesis.

Compared to the alternative approach of using powered burrs, theultrasonically-vibrated tool 1 is significantly more accurate, and doesnot flex when it meets increased resistance, which might causeunacceptable collateral bone damage. Frictional heating is lower withthe tool 1 shown than with powered burrs, and the amount of bone swarfproduced is significantly lower.

Manual (chisel-like) osteotomes require considerable force to drivebetween the prosthesis and the femur, which could damage a weakenedfemur wall and frequently compromises the directional accuracy of thetechnique.

The tool 1 may also be of use in other surgical procedures where rapidand accurate bone cutting is required, such as bone grafting oramputations.

A second osteotomy tool 21 is shown in FIG. 3A. As for the first 1, itcomprises a cylindrical body 2 with a proximally-mounted threaded spigot3 by which it is connectable to a generator of ultrasonic vibrations.However, in this case, the generator produces torsional mode ultrasonicvibrations. As for the longitudinally-vibrated first tool 1, vibrationsin the frequency range 20-75 kHz are preferred.

The second tool 21 is provided with a generally hemicylindrical blade27, aligned coaxially with the connecting body 2 along a longitudinalaxis 18 of the tool 21. A distal tip 28 of the hemicylindrical blade 27is provided with a plurality of teeth 23. The teeth 23 are shown assymmetrical, although they may be asymmetrical as for the teeth 13 ofthe first tool 1, set in either sense or even set in alternating senses.The tip 28 thus comprises a generally semicircular cutting edge, asshown in FIG. 3B.

The torsional mode ultrasonic vibrations transmitted through theconnecting body 2 to the blade 27 thus vibrate the tip 28 as shown byarrows 26. The user rotates the second tool 21 manually about the axis18, without needing to exert significant longitudinal force, and theultrasonic vibrations cause the tool 21 to cut rapidly and accuratelyinto the bone to which it is applied.

As well as being useful for cutting between a prosthesis and a concaveinner wall of a long bone, the second tool 21 may also be usable to cutcircular bone samples, or in cranial surgery. Although a generallyhemicylindrical blade 27 is probably optimal for arthroplasty revisionwork, blades comprising greater or lesser proportions of a hollowcylinder may be appropriate in other applications.

A third osteotomy tool 31, shown in FIG. 4, is a preferred variant ofthe first tool 1, shown in FIG. 1. As for the first tool 1, the thirdtool 31 comprises a connecting body 2 having a threaded spigot 3, bywhich the tool 31 may detachably be connected to a generator ofultrasonic vibrations. An elongate blade portion 4 extends from a distalend of the connecting body 2, generally coaxially aligned therewith.

The blade portion 4 comprises a proximal blade root 5 of generallyrectangular cross-section, linked by a tapered portion 6 to a thin,elongate blade 37 with a generally rounded distal tip 8. As for theblade 7 of the first tool 1, this comprises a distal portion having twooppositely-facing lateral cutting edges 9, 10. A plurality of teeth 13extend along each cutting edge 9,10 and the rounded tip 8 that joinsthem.

The blade 37 of the third tool 31 differs in cross-sectional profilefrom that of the first tool 1. Whereas the blade 7 has a rectangularcross-section, the blade 37 has a substantial bevelled region 32extending longitudinally of the blade 37 adjacent each edge 33 thereofand around its distal tip 8. (A corresponding bevelled region 32 isprovided on a reverse face of the blade 37 to that visible in FIG. 4).

Thus, as shown more clearly in FIG. 5, the blade 37 has an octagonalcross-section. Respective bevelled regions 32 on each face of the blade37 define a narrow edge 33 extending between them. It is preferable thatthe edge 33 is not actually sharpened, to reduce the likelihood of itcutting anything accidentally while the tool 31 is not ultrasonicallyactivated.

As shown in FIG. 6, the indentations between the teeth 13 of the blade37 extend only partially across the bevelled regions 32. They are thusboth triangular in plan view (see FIGS. 2 and 3) and generallytriangular in profile.

The teeth 13 of the blade 7 of the first tool 1 have a substantiallyrectangular cross-section, and it is believed that the outer cornersthereof may be prone to damage. It is probable that an activated tool 1would at some point come into contact with a prosthesis being removedand the corners of the teeth 13 would tend to impact thereon. Therewould be a significant chance of these corners being knocked off,notched or chipped as a result. It is important to balance anultrasonically-vibratable blade, and significant loss of material fromthe teeth 13 might require the whole blade 7 to be rebalanced or evendisposed of. Also, if damage occurs at a region of the blade 7 that isunder raised stress, fatigue fractures of the blade 7 might quicklyfollow, originating from the damage.

The blade 37 with bevelled regions 32 avoids such problems to a greatextent. While a face of the blade 37 might contact the prosthesis inuse, its teeth 13 (and particularly the narrow edge 33 forming the tipsof the teeth 13) are set back from the face and less likely contact theprosthesis. Even if they did, the profile created means that suchcontacts would be more glancing and less liable to cause damage.Nevertheless, the tooth 13 profile of the blade 37 of the third tool 31is just as effective as that of the first tool in cutting throughcancellous bone.

A similar tapered profile may also be created around the cutting distaltip 28 of the second tool 21.

1. A tool adapted to cut bone comprising: means for generatingultrasonic vibrations, and an elongate blade operatively connectablethereto, the blade having at least one cutting edge provided with aplurality of serrations each having a first cutting facet substantiallytransverse to the direction of said ultrasonic vibrations.
 2. A tool asclaimed in claim 1, wherein the means for generating ultrasonicvibrations produces longitudinal mode ultrasonic vibrations directedsubstantially parallel to a longitudinal axis of the blade.
 3. A tool asclaimed in claim 1, wherein the blade comprises an elongatesubstantially planar member having two substantially oppositely-facinglateral edges.
 4. A tool as claimed in claim 3, wherein saidoppositely-facing lateral edges each extend substantially parallel tothe longitudinal axis of the blade.
 5. A tool as claimed in claim 3,wherein the elongate member further comprises a rounded distal tipextending between a distal end of a first of said oppositely-facinglateral edges and a distal end of a second of said oppositely-facinglateral edges.
 6. A tool as claimed in 5, wherein the at least onecutting edge extends along at least part of a respective one of saidoppositely-facing lateral edges.
 7. A tool as claimed in claim 6,wherein the at least one cutting edge extends around all or part of therounded distal tip.
 8. A tool as claimed in claim 3, wherein at leastpart of the blade has a cross-sectional profile tapering towards one oreach of said oppositely-facing lateral edges.
 9. A tool as claimed inclaim 5, wherein the blade has a cross-sectional profile adjacent itsdistal tip tapering towards said distal tip.
 10. A tool as claimed inclaim 1, wherein each serration of the plurality of serrations isgenerally triangular.
 11. A tool as claimed in claim 1, wherein eachserration of the plurality of serrations has its first cutting facetextending generally orthogonally to a local alignment of the cuttingedge.
 12. A tool as claimed in claim 1, whereas the means for generatingultrasonic vibrations produces torsional mode ultrasonic vibrations. 13.A tool as claimed in claim 12, wherein the blade comprises an elongatemember having a curved cross-section substantially comprising an arc ofa circle.
 14. A tool as claimed in claim 13, wherein said tool isadapted to be torsionally vibratable about a longitudinal axis extendingthrough the centre of said circle.
 15. A tool as claimed in claim 13,wherein a distal tip of the elongate member comprises the at least onecutting edge.
 16. (canceled)
 17. A method of cutting bony materialcomprising the steps of: providing a tool comprising: means forgenerating ultrasonic vibrations, and an elongate blade operativelyconnectable thereto, the blade having at least one cutting edge providedwith a plurality of serrations each having a first cutting facetsubstantially transverse to the direction of said ultrasonic vibrations,applying said at least one cutting edge of the tool to a surface of bonymaterial to be cut, causing the tool to vibrate at an ultrasonicfrequency by actuating the means for generating ultrasonic vibrations,and drawing the cutting edge of the tool across said surface.